I think she was a bit young to be buying her own clothes when she left Kanto, so probably wouldn't have had quite enough exposure to the experience to know prices all that well? I think either way, though, the logic chain makes sense in her mind. She might've been hoping it was a bit cheaper than a normal yukata as well? She can absolutely be talked into using Lord Bastille's money for it, but left to her own devices she'd probably just do a 'that'd be nice...' then move on.